The catalytic hydrogenation of DAHNP is described, for example, in U.S. Pat. No. 2,447,523 and in German Offenlegungsschrift 36 38 635. In accordance with the U.S. patent, a 0.4 to 0.5 molar suspension of DAHNP is hydrogenated at 1.4 to 2.3 bar in 0.1 to 1.0N sodium hydroxide (corresponding to 0.2 to 2.5 mols sodium hydroxide per mol DAHNP) in the presence of palladium, platinum oxide or Raney nickel as a catalyst. This process, however, has several disadvantages. First, the concentration of DAHNP is limited to 0.4 to 0.5 mol/liter, since higher concentrations cause a rapid thickening of the reaction mixture, thus preventing any efficient mixing which in turn greatly slows down the hydrogen uptake. Secondly, the required amount of catalyst is high, reaching 50 g of 5 wt.-% palladized charcoal (corresponding to 2.5 g metallic palladium), 4 g platinum oxide or 150 g Raney nickel per mol DAHNP.
German Offenlegungsschrift 36 38 635 eliminates both of these disadvantages in that DAHNP is hydrogenated under a hydrogen pressure of 1.4 to 21 bar in aqueous solution at 20.degree. to 80.degree. C. and a concentration of 0.5 to 3.0 mol/liter in the presence of 0.02 to 0.2 g metallic palladium (1 to 10 wt.-% on charcoal) per mol of DAHNP while metering in 0.8 to 1.5 mols of sodium hydroxide per mol of DAHNP during the hydrogenation.
Despite these important advantages, the process of the German reference still has the disadvantage that it is, for the most part, performed in the alkaline range. However, with as little as 0.2 mol of alkali per mol of DAHNP, which is the smallest amount of alkali according to the U.S. patent, the pH rises to 12 and, according to the German reference, with ammonia it rises to 10 and more. Attempts to hydrogenate DAHNP in aqueous acid solution failed according to column 1, paragraph 3, of U.S. Pat. No. 2,447,523.
Moreover, we have found that the hydrogenation in the alkaline range has the great disadvantage that significant amounts (up to more than 50%) of the expensive catalytically active noble metals go into solution during the hydrogenation. When the catalyst is then removed by filtration, these noble metals are not recovered and move into the production waste water and into the product TAHP-sulfate from which they cannot be recovered with an acceptable amount of cost and labor. Because of the high price of the noble metals which are used, already small losses negatively affect the economy of the process. Furthermore, the contamination of the TAHP-sulfate, which is further processed into pharmaceutically useful substances, such as guanine and folic acid, is very questionable. Also, the ecological effects of these metals on creeks, rivers and lakes are largely unknown.